I. Field
The following description relates generally to communication systems, and more particularly to multiple-user uplink communication in a wireless network.
II. Background
To address increasing bandwidth requirements of wireless communications systems, different schemes are being developed to allow multiple access terminals to communicate with a single access point by sharing the channel resources while achieving high data throughputs. Multiple Input or Multiple Output (MIMO) technology represents one such approach that has recently emerged as a popular technique for the next generation communication systems. MIMO technology has been adopted in several emerging wireless communications standards such as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. IEEE 802.11 denotes a set of Wireless Local Area Network (WLAN) air interface standards developed by the IEEE 802.11 committee for short-range communications (e.g., tens of meters to a few hundred meters).
In wireless communications systems, medium access (MAC) protocols are designed to operate to exploit several dimensions of freedom offered by the air link medium. The most commonly exploited dimensions of freedom are time and frequency. For example, in the IEEE 802.11 MAC protocol, the “time” dimension of freedom is exploited through the CSMA (Carrier Sense Multiple Access) protocol. The CSMA protocol attempts to ensure that no more than one transmission occurs during a period of potential high interference. Similarly, the “frequency” dimension of freedom can be exploited by using different frequency channels.
Recent developments have introduced space as a dimension to be exploited to increase, or at least more efficiently use, existing capacity. Spatial Division Multiple Access (SDMA) can improve utilization of the air link by scheduling multiple access terminals for simultaneous transmission and reception. Data is sent to each of the terminals using spatial streams. For example, with SDMA, a transmitter forms orthogonal streams to individual receivers. Such orthogonal streams can be formed because the transmitter has several antennas and the transmit/receive channel consists of several paths. Receivers may also have one or more antennas (MIMO, SIMO). For this example, it is assumed that the transmitter is an access point (AP) and the receivers are access terminals (ATs). The streams are formed such that a stream targeted at AT-B, for example, is seen as low power interference at other access terminals (e.g. AT-C, AT-D, . . . , etc.). Such a targeted stream will not cause significant interference at other ATs and are likely ignored. To form these orthogonal streams, the AP needs to have channel state information (CSI) from each of the receiving ATs. Although CSI can be measured and communicated in several ways, thereby adding complexity, the use of CSI optimizes the configuration of SDMA streams.
Additional complexities arise when MIMO is applied to multi-user (MU) systems. For example, typically, the AP controls the uplink (UL) communication process. However, in certain configurations, uplink scheduling requires that ATs contend with the AP for channel access. In other words, the AP will act as an additional AT trying to gain access to the transmission medium, thereby affecting all ATs attempting to gain access. Further, the ability of the AP to efficiently schedule UL-SDMA traffic depends on knowledge of the amount of uplink data available at ATs to be served. Improvements to current UL scheduling schemes and mechanisms to share information useful for scheduling are desirable.